Pressure sensors can be used to measure pressure variations in air as well as any other gas. The pressure variations in a sample gas are measured against known values of pressure variations in air. These pressure variations are an important parameter in determining various properties of the sample gas. Pressure sensors that measure such pressure variations find widespread application in areas such as industrial process control, automobile engine monitoring and various biomedical processes.
One widely used technique for measuring pressure variations is the ‘hot-wire’ technique. Pirani gauges employ this technique for measuring pressure variations in air. Pirani gauges operate by heating a wire by passing current through it, and monitoring the resulting changes in the resistance of the wire. The resistance in the wire is a function of the wire temperature, and the wire temperature is a function of the cooling rate of the wire, which in turn is a function of the ambient pressure. Thus, changes in resistance of the wire are measured in order to determine the changes in pressure. However, thermal conductivity of air is almost constant from atmospheric pressure to around 10 mBar. Hence, the pirani gauge is only useful for measuring pressure variations below approximately 10 mBar.
Another technique used to sense pressure variations uses a silicon based pressure sensor. The two commonly used types of silicon-based pressure sensors are piezoresistive and capacitive pressure sensors. Both the piezoresistive and capacitive pressure sensor uses a thin silicon diaphragm, which is fabricated by anisotropic etching of a thicker silicon substrate. In the piezoresistive pressure sensor, pressure variations in one side of the diaphragm results in a flexure, which is directly proportional to the applied pressure. The piezoresistive type of pressure sensor relies on stress-induced changes in the resistivity of diffused resistors in the diaphragm. The capacitive sensor relies on capacitive variations between the diaphragm and a metal plate, which is placed parallel and close to the diaphragm. Further, both types of the pressure sensors require the silicon chip attached to the metal plate to hermetically seal the chamber on one side of the diaphragm. However, the requirement of hermetic sealing increases the cost of the pressure sensor as the sealing is performed on an individual chip rather than on a complete wafer level.
Another type of pressure sensor employs the concept of ionization to sense pressure variations. The ionization gauge involves a heating filament that emits electrons which ionizes gas molecules. An ion collector then captures the ions. The amount of gas molecules available to be ionized decreases as the gas pressure decreases. This ultimately results in lesser ions to be received by the ion collector. However, the ionization gauge is effective only at low pressures (around at 0.00013 mBar). Further, several devices and electrical connections required by the ionization gauge burn out at atmospheric pressures.
One or more of the above-mentioned pressure sensors suffer from at least one disadvantage. Hence, there is a need of a pressure sensor that is operable in the pressure ranges below the atmospheric pressure. Further, the pressure sensor should have a good sensitivity and resolution to the pressure variations. Also, there is a need of significant cost reduction in the fabrication of the pressure sensor with respect to the available techniques.